1. Field of the Invention
The invention relates to a process for forming a protective coating on composite metals of the type generally referred to in the art as "phosphate conversion coatings." The coating layer protects the underlying metal from corrosion and primes the surface to improve the adhesion of subsequently applied protective or decorative coatings.
2. Discussion of Related Art
Conventional pre-treatment of metal surfaces is typically designed to protect the surfaces from corrosive attack of the environmental atmosphere for an extended period of time and to improve adhesion of subsequent coatings. The most widely used commercial methods of pre-treatment are the formation of phosphate and chromate conversion coatings, with the selection of the appropriate method usually depending on which of the two methods will provide better results in a particular case. Futther criteria for the selection of the treatment procedure to be employed include the coatings to be subsequently applied, the contemplated further treatment of the metal substrate, its size, and the quality specifications which must be met.
Heretofore, it has been common practice in the art to employ different types of coating processes for each kind of metal to be treated. While these different processes permit specific quality standards to be met for high-quality products, in practice these separate processes have the inherent serious drawback of requiring a great deal of space. Moreover, separate phosphating or chromating processes require the solutions or baths to be freshly prepared before use, so that unused solutions or baths have to be discarded or--if possible at all--to be stored and topped up by the addition of new chemicals for a renewed use. This is wasteful of both space and materials and is impractical on both economic and ecological grounds.
It is known from the prior art to provide metals, including composite metals, with a phosphate coating which protects the underlying metal from corrosion and improves the adhesion of subsequent coatings. Steel, galvanized metal, and aluminum are all routinely protectively coated by processes such as zinc phosphation. However, a determination of the operating of the pretreatment bath constitutes a problem when composite metals are to be pretreated. For example, the formation of an opaque coating on certain metals requires certain coating bath components to be employed which in turn renders coating of the other metals present more difficult, if not impossible. In order to obtain a phosphate coating meeting minimum requirements on aluminum, the addition of fluoride (for example in the form of acidic bifluorides, fluoroborate or fluorosilicate) to the phosphating bath is indispensable; this additive, however, adversely affects the coating quality on other metals present, such as electrogalvanized steel. In fluoride-free multimetal conversion coating processes, good protection from corrosion can be obtained on steel and galvanized metals; however, the results are not satisfactory on aluminum. Accordingly, on steel and galvanized steel the specifications of the German automotive industry for the quality class I (480 hours salt spray test; infiltration less than 2 mm) can be readily met in a fluoride-free process, whereas on aluminum they can not, and high-quality protectively coated composites of steel and aluminum are consequently difficult to obtain.
The protective pre-treatment of metal surfaces of steel, galvanized metal, and aluminum is also affected with "non-coating processes" wherein aqueous alkali or ammonium orthophosphate solutions containing surfactants are used. However, these processes only coat steel surfaces, with formation of mixed iron phosphates. Galvanized surfaces or aluminum surfaces treated with such solutions are only cleaned and degreased. Thus, the protection from corrosion is distinctly inferior to that effected by a zinc phosphate coating, and regularly barely meets the standards of the German automotive industry quality class II (240 hours salt spray test; infiltration less then 3 mm).
Meanwhile, today in all industrial fields to an increasing degree, surfaces are varnished and coated using powder processes which require the anticorrosive coatings obtained to be extremely thin films. Multimetals treated by these processes do not comply with standards for each metal to the same degree.